1. Field of the Invention
The invention is related to off-axis multi-channel fiber optic slip ring to provide transmission of data in optic form between a mechanically rotational interface with a through bore.
2. Description of Related Art
It is well known that the devices to transmit optical data between two independently rotational members are called fiber optical rotary joints, or optical slip ring. There are single channel, two channel and multi-channel fiber optical rotary joints. However, most of them are categorized as on-axis fiber optical rotary joint because the optical paths are located along the axis of rotation, or occupy the central space along the axis of rotation. If the central space along the rotational axis is not accessible, the optical light paths would not be allowed to path through the central area along the rotational axis. Such devices are usually called off-axis optical slip ring.
The simplest, off-axis slip ring has been described in U.S. Pat. No. 4,492,427, which comprises two opposed annular fiber bundles and increasing the number of such concentric annular bundles radially would make the device multi-channeled. The concentric, annular fiber bundle fiber optic slip rings are bi-directional but do have a modulated light loss dependent on the rotational angle. For minimizing the importance of the modulation, a digitized signal rather than an analog signal has to be used. This off-axis slip ring only could be used for multi-mode fibers, not single mode fibers.
U.S. Pat. No. 4,460,242 discloses an optical slip ring employing optical fibers to allow light signals applied to any one or all of a number of inputs to be reproduced at a corresponding number of outputs of the slip ring in a continuous manner. It includes a rotatable output member, a stationary input member and a second rotatable member which is rotated at half the speed of the output member like a de-rotator. The input member having a plurality of equispaced light inputs and the output member having a corresponding number of light outputs and the second rotatable member having a coherent strip formed of a plurality of bundles of optical fibers for transmitting light from the light inputs on the input member to the light outputs.
Another U.S. Pat. No. 4,943,137 assume the similar idea, where, a de-rotating, transmissive intermediate optical component with an array of lensed optical transmitters and receivers respectively mounted on the rotor and stator. The derotating, intermediate optical component comprises an image conduit, image transporter, or coherent optical fiber bundle of close-packed monofibers or multifibers.
But actually, it is almost no way to handle and arrange so many fibers on the said rotatable members, especially for large diameter slip ring. The optical loss is very obvious for multi-mode fibers. It is almost impossible to use single mode fibers. The effect of damaged fibers, the presence of debris, separation distances, component tolerances, or backlash in the gearing also cause problems.
A more sophisticated approach can be found in U.S. Pat. No. 6,907,161. The patent uses multiple inputs and pick-ups to send and receive data across members that have large diameters. The use of multiple inputs and pick-ups is required to keep the optical signals at a level that is sufficiently high to permit the photodiode receivers to operate. Wave guides are employed. The multiple inputs and pick-ups also cause a rapid rise and fall of the signal because the signal reflects from one area of the waveguide to another. The drawback is to use photodiode receivers which is an electro-optical device, so that the output signal is electrical and the power must be high. Besides, there is a time jitter thus limiting the data rate.